The term “Semantic Web” is often used more specifically to refer to the formats and technologies that enable it. The collection, structuring and recovery of linked data are enabled by technologies that provide a formal description of concepts, terms, and relationships within a given knowledge domain. These technologies are specified as W3C standards and include:
Resource Description Framework (RDF), a general method for describing information
RDF Schema (RDFS)
Simple Knowledge Organization System (SKOS)
SPARQL, an RDF query language
Notation3 (N3), designed with human-readability in mind
N-Triples, a format for storing and transmitting data
Turtle (Terse RDF Triple Language)
Web Ontology Language (OWL), a family of knowledge representation languages
Rule Interchange Format (RIF), a framework of web rule language dialects supporting rule interchange on the Web
The Semantic Web Stack illustrates the architecture of the Semantic Web. The functions and relationships of the components can be summarized as follows:
XML provides an elemental syntax for content structure within documents, yet associates no semantics with the meaning of the content contained within. XML is not at present a necessary component of Semantic Web technologies in most cases, as alternative syntaxes exists, such as Turtle. Turtle is a de facto standard, but has not been through a formal standardization process.
XML Schema is a language for providing and restricting the structure and content of elements contained within XML documents.
RDF is a simple language for expressing data models, which refer to objects (“web resources”) and their relationships. An RDF-based model can be represented in a variety of syntaxes, e.g., RDF/XML, N3, Turtle, and RDFa. RDF is a fundamental standard of the Semantic Web.
RDF Schema extends RDF and is a vocabulary for describing properties and classes of RDF-based resources, with semantics for generalized-hierarchies of such properties and classes.
OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. “exactly one”), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.
SPARQL is a protocol and query language for semantic web data sources.
RIF is the W3C Rule Interchange Format. It’s an XML language for expressing Web rules which computers can execute. RIF provides multiple versions, called dialects. It includes a RIF Basic Logic Dialect (RIF-BLD) and RIF Production Rules Dialect (RIF PRD).
Current state of standardization
Uniform Resource Identifier
Web Ontology Language (OWL)
Rule Interchange Format (RIF)
Not yet fully realized:
Unifying Logic and Proof layers